回火处理对Q420建筑用钢显微组织与力学性能的影响

Effects of Tempering Treatment on Microstructure and Mechanical Properties of Q420 Building Steel

对控轧控冷态Q420钢板进行了不同回火温度和时间的热处理试验,研究了回火对试验钢拉伸性能和显微组织的影响。结果表明,回火温度为450~600℃时,随着回火保温时间从0~120 min,试验钢的抗拉强度先减小后上升,屈服强度和屈强比均呈现逐渐上升的趋势;在回火温度为550℃时,试验钢具有较高的抗拉强度和屈服强度及较低的屈强比。控轧控冷态试验钢的显微组织为针状铁素体+M/A岛+退化珠光体。550℃回火保温不同时间后,试验钢的组织类型未发生改变,但当回火时间≥60 min,基体组织中铁素体晶粒粗化,且退化珠光体中渗碳体形态有向短片状转变的特征,基体组织中的M/A岛含量和平均尺寸随着保温时间延长而都逐渐减小。550℃/60 min回火处理后,试验钢基体组织中的M/A岛已发生明显分解,位错密度减小,且基体中析出了较多的尺寸约在10~25 nm的(Nb,Ti)C强化相。

The heat treatment tests under different tempering temperature and time were carried out on the cold controlled Q420 steel plate.The effects of tempering on tensile properties and microstructure of controlled cold Q420 steel plate were studied.The results show that the tensile strength of the test steel decreases and then increases when the tempering temperature is 450-600℃,and the yield strength and yield-strength ratio both show a gradual upward trend with the holding time of tempering from 0 to120 min.At a tempering temperature of 550℃,the test steel has higher tensile strength and yield strength and lower yield strength ratio.The microstructure of controlled cold test steel is needle ferrite M/A island degraded pearlite.After tempering at 550℃for different time,the microstructure of the test steel is not changed,but when the tempering time is more than 60 min,the grain size of the matrix is coarsened.The morphology of the degraded pearlite is characterized by a short-sheet transition,and the M/A island content and the average size of the matrix decrease gradually with the increase of the holding time.After tempering at 550℃/60 min,the M/A islands in the matrix of the tested steel have been obviously decomposed.The dislocation density decreases,and more sized(Nb,Ti)C strengthening phases of about 10-25 nm are precipitated in the matrix.